If your 401(k) plan allows after-tax contributions, you can roll that money over to a Roth IRA when you leave the company

I have read recent articles on making after-tax contributions to a 401(k) plan and later rolling them into a Roth IRA. I work for a company that offers the standard pretax 401(k) and the after-tax Roth 401(k). How do I go about putting after-tax money [into my standard 401(k)] so that, say, 10 years from now, when I leave the company, I can roll all my after-tax money into my Roth at a securities firm?